1. Field of the Invention
The present invention relates to epoxy molding compositions. More particularly, the present invention relates to epoxy molding compositions which are particularly useful as optoelectronic encapsulants for light emitting devices.
2. Brief Description of Related Technology
Epoxy resin compositions are widely used for electronic packaging materials in the electronics industry and, in particular, as encapsulants for semiconductor elements and electronic circuits. Accordingly, epoxy resin-based compositions have been widely used in the formation of molding compositions for use as electronic packaging materials. Transparent epoxy molding compositions are well known for use as encapsulants in connection with optoelectronic devices, such as light-emitting diodes (LEDs), for use for lighting applications in the electronics industry.
LEDs have traditionally been multilayer epitaxial structures based mainly on AlGaAs and InGaAlP as basic materials, Such LEDs are typically encapsulated in a transparent epoxy composition, which is typically provided in a dome shape for efficient transmission of light from the LED.
Epoxy compositions useful in encapsulating optoelectronic devices such as LEDs may involve liquid-type compositions, which can be filled or poured about an LED and cured. Additionally, partially cured epoxy compositions may be used for encapsulating LEDs, in which an epoxy composition is partially cured or “B-staged” to form a “pellet”, which can then be transfer molded with the electronic device under heat and pressure to encapsulate the LED.
More recently, GaN (gallium nitride)-based semiconductor devices have been used in UV/blue lasers and UV/blue LEDs. UV/blue LEDs are LEDs which are capable of emitting in the ultraviolet (UV) range, or in the blue range, or in both the UV and blue ranges of the electromagnetic spectrum. Conversion of UV/blue light from an LED to visible white light can be accomplished using phosphors, which are typically applied to the surface of the LED and then encapsulated with an epoxy encapsulant.
U.S. Pat. No. 5,886,401 to Liu discloses an LED structure including a transparent epoxy compound for encapsulating the LED, and including a phosphor layer coated on the outer surface of the epoxy compound. Such an arrangement requires additional manufacturing steps for providing the separate layers of epoxy and phosphor, which results in additional time and cost for manufacturing.
U.S. Pat. No. 5,149,971 to McEihaney et al. discloses a scintillator assembly which includes a body of optically-transparent epoxy and an amount of phosphor particles embedded within the body adjacent one surface. The phosphor particles are mixed within the liquid epoxy when in an uncured state and settle to the bottom surface of a mold in which the liquid epoxy is contained. Upon curing into a hardened body, the phosphor particles are coated on one surface thereof.
U.S. Pat. No. 5,813,753 to Vriens et al. discloses an LED for use in lighting applications which includes a UV/blue LED located in a depression of a cup, with the cup being filled with a light transmitting cycloaliphatic epoxy encapsulant with phosphor particles dispersed in the encapsulant or adhered to the surface of the LED. Such cycloaliphatic epoxy encapsulants are well known for use as liquid epoxy compositions in encapsulating electronics. Such liquid cycloaliphatic epoxy compositions, however, cannot be B-staged due to their inherent high reactivity. Moreover, because of their low viscosity, the phosphor particles typically settle out of the liquid composition during curing, thereby providing a non-uniform distribution of phosphor throughout the cured encapsulant.
Such epoxy compositions should be as resistant to heat and UV light as possible to help prevent degradation of the epoxy composition during operation of the light emitting device.
Accordingly, it would be advantageous to provide a molding composition useful as an optoelectronic encapsulant for LEDs which provides for substantially uniform distribution of phosphors and which is easy to manufacture and use as a molding composition. It would also be advantageous to provide a molding composition useful as an optoelectronic encapsulant for LEDs having improved resistance to heat and UV light.